While GCC enables int-conversion warnings by default, it will currently
only warn about such errors even in case where "-DENABLE_WERROR=ON" has
been passed to CMake. Explicitly enable int-conversion warnings by using
our `ENABLE_WARNINGS` macro, which will automatically use
"-Werror=int-conversions" in case it has been requested by the user.

(cherry picked from commit aa0ae59a)